Instruments such as oscilloscopes, spectrum analyzers, modulation analyzers, and vector signal analyzers, normally implement a trigger mechanism to align the beginning of a viewing and analysis interval with a particular instant in time. This desired trigger time may be identified by a separate “trigger input” signal. It may also be based on an attribute of the input signal such as the crossing of a particular voltage or power level. Virtually all of these instruments enable the selection of the type or source of the trigger signal and the related signal attributes such as threshold and polarity.
The precision of the trigger timing is complicated by the fact that most instruments convert the input signal to digital form by sampling the input signal at uniform time intervals. If the desired trigger time does not precisely align with one of these samples, there is an error in the trigger timing. Unless additional measures are taken, the inherent trigger uncertainty is one period of the instrument's sample clock. In some cases, this amount of uncertainty can be tolerated. However, there are applications that require greater precision.
For example, analysis of a periodic signal or a cyclostationary signal (a signal with periodic statistics) typically involves displaying or processing ensembles of many signal periods according to accurate time alignment. An ensemble, as used herein, refers to samples of a single period of a signal. An example of analysis of multiple ensembles of signal data is coherent averaging of several periods of a signal to reduce measurement noise while retaining the underlying signal. The trigger precision requirements for periodic or cyclostationary processing often exceed the inherent limit imposed by an instrument's sample rate.
Some instruments provide a trigger interpolator which measures the time interval between an externally supplied trigger and the next available signal sample. The time offset is then used to post-process the captured data to effectively shift the time alignment to correspond to the desired trigger time. This technique has been observed to improve the effective trigger accuracy by a factor of 10 to 100. However, this technique involves added complexity for the trigger interpolation functionality. Additionally, the post-processing time can be significant. Furthermore, a degree of inaccuracy remains in the time alignment.
Techniques have been developed to extract high resolution trigger timing by post-processing the data even when an external signal is not supplied. Because the value of the offset between the sampling timing and the trigger timing is determined from the signal itself, these techniques can only be applied in a limited number of special cases. After the offset is determined, the timing offset is compensated using time consuming post-processing techniques. However, the extraction of the trigger offset can artificially skew the timing and obscure jitter effects that are present in the signal. Also, in low signal-to-noise conditions, this technique cannot be applied.